The present invention is broadly concerned with a spinal fusion cage system. More particularly, it is directed to an articulated implant which can be selectively anteriorly expanded in situ to form a wedge with an adjustable angle of inclination for supporting adjacent vertebrae in normal curved alignment.
The spine is a column of stacked vertebrae which are normally axially aligned along the median plane. Thus, when viewed from the front or back the spine appears to be straight. When viewed from a lateral perspective, however, the spine is shown to be comprised of four distinct curves. Each vertebra is angularly displaced in the coronal plane in accordance with its position along one of the respective curves.
The structure of each vertebrae includes a rounded, weight bearing anterior element, or vertebral body, which is separated from the adjacent superior and inferior vertebral bodies by fibrocartilage pads or discs. These intervertebral discs support the adjacent vertebrae in an appropriate angular orientation within a respective spinal curve and impart flexibility to the spine so that it can flex and bend yet return to its original compound curvate configuration.
When aging, injury or disease cause damage to the discs or to the vertebrae themselves, it may be necessary to surgically remove a disc and fuse the adjacent vertebral bodies into a single unit. Such surgical arthrodesis is generally accomplished by implanting a cage-like device in the intervertebral space. The cages are apertured, and include a hollow interior chamber which is packed with live bone chips, one or more gene therapy products, such as bone morphogenic protein, cells that have undergone transduction to produce such a protein, or other suitable bone substitute. Following implantation, bone from the adjacent vertebrae eventually grows through the apertures and fuses with the bone of the adjacent vertebrae above and below the cage, thus fixing the adjacent bones as well as the cage in position.
Once the disc has been removed from the intervertebral space, the angular orientation of the adjacent vertebrae is established and stabilized by the three dimensional geometry of the implanted fusion cage, and the vertebrae will eventually fuse in this orientation. The cervical and lumbar curves each present a region of normal anterior convexity and posterior concavity or lordosis. There is a need for a posterior implant for use in these regions, and in particular in the lumbar region, which can be adjusted in situ to achieve and maintain normal lordosis of the vertebrae.
Previous attempts to achieve normal spinal curvature with fusion cages have involved trial insertion of cages of various different sizes into the intervertebral space, removal of the cage and replacement with another until an optimal angular incline is achieved. There is a need for a modular and articulated implant which can be installed between the vertebrae in a first configuration, and adjusted in situ into a wedge configuration. It is also preferable that the bearing surfaces of the device be supported at both the anterior and the posterior end, so that neither end of the device is left unsupported following angular adjustment.
Once installed in an intervertebral space, a spinal implant may be subject to twisting forces caused by unequal lateral distribution of weight on the adjacent vertebral bodies. This may occur, for example, during normal bending and reaching activity. There is also a need for an implant which will provide torsional stability to resist such twisting forces.
The apparatus of the present invention is specifically designed to provide a modular intervertebral implant which can be selectively expanded anteriorly in situ to form a wedge which stabilizes the adjacent vertebrae in normal curved alignment without loss of torsional stability.
The present invention is directed to an apparatus and method for implanting an articulated modular cage in the intervertebral space and adjusting it in situ to support the adjacent vertebrae in a normal curved alignment while permitting fusion of the adjacent bones. The fusion cage system of the present invention includes a base unit having a pair of sockets for receiving a pair of corresponding legs. The sockets permit pivotal movement of the legs about a posterior fulcrum from a closed, parallel insertion position to an anteriorly open, wedge-shaped orientation which may be selectively adjusted to provide appropriate angular support. The base also includes a threaded bore for receiving a wedge or driver. The driver is inserted through the bore and is operable to urge the opposed anterior ends of the legs apart while pivoting the posterior ends within their respective sockets. A plurality of drivers having elongate shafts with tips of various diameters is provided. A driver is selected and advanced between the opposed ends of the legs to support the legs in a predetermined angular orientation. Preferably, each leg unit includes a driver-receiving groove on the inner surface of the opposed end for providing torsional stability. Pins may also be employed to impart torsional stability. Alternatively, the base may be of integral construction with one of the legs, and may include a socket for receiving and permitting pivotal rotation of the second leg to form a wedge.
The principal objects of the present invention are: to provide an improved apparatus and method for fusing together adjacent vertebrae; to provide such an apparatus and method for introducing a bone graft between adjacent vertebrae; to provide such an apparatus and method for intervertebral implantation while maintaining or correcting the angular alignment of the vertebrae of the spine; to provide such an apparatus and method for implanting a dual cage system; to provide such an apparatus and method for adjustment of the alignment and balance of the spine in situ; to provide such an apparatus having an intervertebral cage which can be selectively anteriorly expanded to form a wedge with an adjustable angle of inclination; to provide such an apparatus having two such independently adjustable intervertebral cages; to provide such an apparatus having a modular intervertebral cage system; to provide such an apparatus having an articulated intervertebral cage; to provide such an apparatus having a pair of legs and a base permitting articulated movement of one or both of the legs about a posterior fulcrum from a closed, parallel insertion position to an anteriorly open, wedge-shaped orientation; to provide such an apparatus having an array of drivers with tips of various diameters that each provides a different degree of expansion to allow for variation in the angular configuration between the top and bottom of the cage for adjustment of the angle of curvature between two vertebral bodies; to provide such an apparatus having an intervertebral cage including structure providing torsional stability; to provide such an apparatus having a groove on the inner anterior surface of the leg for receiving and retaining the tip of a driver; to provide such an apparatus having a pair of pins coupled with the anterior bearing surface of one of the legs for adjustable registry with a pair of corresponding apertures on the second leg; to provide such a fusion cage which includes an interior chamber for supporting a bone graft; to provide such a fusion cage which is apertured to permit outgrowth of a bone graft into the surrounding vertebrae; to provide a method for using such an apparatus for implanting a cage unit between two adjacent vertebral bodies, packing the cage unit with a bone graft, adjustably coupling the cage unit with a driver for forming the cage unit into a wedge having a predetermined angle between top and bottom surfaces thereof, and permitting the bone graft to grow and fuse the adjacent vertebral bodies together; providing such an apparatus and method which are relatively easy to use, inexpensive to produce and particularly well-suited for their intended usage.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.